Dictionary

Construct a Dictionary

a = {"first": "John", "last": "Smith"} # {'first': 'John', 'last': 'Smith'}

# from a list of tuples
b = dict([("first", "John"), ("last", "Smith")])

# from keyword arguments
c = dict(first="John", last="Smith")

# from dictionary comprehension
d = {x: x**2 for x in range(10)} # {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

Loop Through Items

a = {"domain": "literank.com", "services": 29, "year": 2017}
for k, v in a.items():
    print(k, '->', v)

Add/Update Item

a = {"first": "John", "last": "Smith"} # {'first': 'John', 'last': 'Smith'}
a["age"] = 26
a["first"] = "Tom"
print(a) # {'first': 'Tom', 'last': 'Smith', 'age': 26}

Check If Key Exists

a = {"first": "John", "last": "Smith"}
print("first" in a) # True
print("age" in a) # False

Dictionary's Common Methods

fromkeys

a = ['a', 'b', 'c']
b = dict.fromkeys(a, 10) # 10 is the default value for all keys
print(b) # {'a': 10, 'b': 10, 'c': 10}

clear

a = {"first": "John", "last": "Smith"}
a.clear()
print(a) # {}

get

a = {"first": "John", "last": "Smith"}
print(a.get("first")) # John
print(a.get("age", 28)) # 28

get's function signature is get(key[, default]). Return the value for key if key is in the dictionary, else default. If default is not given, it defaults to None, so that this method never raises a KeyError.

setdefault

a = {"first": "John", "last": "Smith"}
a.setdefault("first", "Tom")
a.setdefault("age", "12")
print(a) # {'first': 'John', 'last': 'Smith', 'age': '12'}

setdefault's function signature is setdefault(key[, default]). If key is in the dictionary, return its value. If not, insert key with a value of default and return default. default defaults to None.

Unpack a Dictionary

def passport_check(first, last):
    print(f"{first} {last} is ok to go!")

a = {"first": "John", "last": "Smith"}
# unpack with double asterisks
passport_check(**a) # John Smith is ok to go!

Code Challenge

Try to modify the code provided in the editor to swap key-value pairs in the dictionary.

e.g. {'a': 'b'} to {'b': 'a'}